The cracked effluent may be cooled in a heat exchanger connected to the furnace cracked gas outlet by a transfer line, which is thus termed a transfer line exchanger (TLE). Conventionally, the cracked gas from many reaction tubes is manifolded, passed into the expansion cone of a TLE, then through a tube sheet and into the cooling tubes of a multitube shell and tube TLE in order to cool the gas and generate steam.
In conventional TLE's the cracked gas is distributed to the cooling tubes by the inlet chamber. Since the cross sectional area of the TLE tubesheet is large compared to the area of the inlet nozzle and outlet collection manifold, the cracked gas must expand when leaving the manifold and contract again when entering the cooling tubes. In a typical exchanger, the velocity drops from 450 ft/sec at the inlet nozzle to 60 ft/sec before entering the cooling tubes. Once in the cooling tubes, the velocity is increased again to approximately 300 ft/sec; this expansion and contraction of the cracked gas coupled with its low velocity in the exchanger inlet chamber causes turbulence and uncontrolled residence time. This uncontrolled residence time causes a deterioration in the selectivity to desirable olefins, and coking. The heavier components and poly-nuclear aromatics in the cracked gas condense and polymerize to form coke in the inlet chamber. During process upsets or onstream decoking, this coke spalls and plugs the exchanger tubes causing a drastic increase in the exchanger pressure drop. Also, when hot gas strikes the dead flow zone caused by the tube sheet between the cooling tubes, heavier components and poly-nuclear aromatics suspended in the cracked gas are knocked out of the gas stream and condense and polymerize to form coke on the tube sheet between the cooling tubes. This coke deposit grows and gradually covers or blocks the entrance to the cooling tubes thus impeding heat transfer and causing the exchanger to lose its thermal efficiency. Furthermore such expansion and contraction of the cracked gas caused by large changes in velocity results in pressure loss, as discussed in U.S. Pat. No. 3,357,485. According to the present invention, these conditions are avoided and pressure loss is reduced.
In the conventional design there is a dramatic increase in velocity (when the gas enters the cooling tubes) which results in that the kinetic pressure loss is great as compared with a small static pressure gain to give an overall much greater pressure loss, as contrasted with the present invention in which there is no large or sudden increase in velocity so that the smaller loss in kinetic pressure as compared with the gain in static pressure gives an overall small pressure loss. Any decrease in velocity along the path of flow is gradual and relatively small as against the standard expansion cone, or velocity may be constant.
The flared expansion chamber is described in the following U.S. Pat. Nos.:
______________________________________ 3,357,485 3,763,262 3,449,212 3,910,347 3,456,719 4,078,292 3,552,487 4,097,544 3,574,781 4,151,217 ______________________________________
In U.S. Pat. No. 3,671,198 the outlet of each reaction tube is connected to a respective quench tube which is surrounded by a cooling jacket. This has the serious drawback that with a single quench tube fitted to a single reaction tube, in the event of plugging of the quench tube by coke, there will be loss of flow and subsequent failure of the reaction tube since the cracked gas will remain therein, will reach excessively high temperature and cause burnout. On the contrary, the subject heat exchange unit has at least two flow paths for the gas and the probability of both becoming plugged simultaneously is very low. This is an excellent safety feature.
As residence time and hydrocarbon partial pressure are decreased and cracking is carried out at higher radiant coil outlet temperatures, the selectivity to desirable olefins is improved. Accordingly, in recent years attention has been directed to the use of pyrolysis tubes affording short residence time, see for example an article entitled "Ethylene" in Chemical Week, Nov. 13, 1965 and U.S. Ser. No. 301,763 filed Sept. 14, 1981, of A. R. DiNicolantonio and V. K. Wei.
To capitalize on the benefits of very low residence time cracking, it is necessary to quench the effluent as quickly as possible in order to stop undesirable cracking reactions. To accomplish this, it is necessary to place the TLE as close as possible to the fired coil outlet to reduce the unfired residence time, i.e., the residence time measured from when the cracked process gas leaves the fired zone of the furnace to when it enters the TLE cooling tubes. It is also desirable to minimize turbulence and recirculation of the cracked gas between the fired outlet and TLE cooling tubes as this uncontrolled residence time causes a deterioration in the selectivity to desirable olefins and polymerization of the heavier components to coke. That is, the uncooled transfer line constitutes an adiabatic reaction zone in which reaction can continue, see The Oil and Gas Journal, Feb. 1, 1971.
It is highly desirable to reduce pressure build-up in the exchanger and loss of thermal efficiency. To accomplish this the dead flow zones between individual cooling tubes must be eliminated to prevent the heavy components in the cracked gas from condensing on these areas and eventually restricting cracked gas flow to the cooling tubes. These dead flow zones between the cooling tubes are not entirely eliminated by the devices described in U.S. Pat. No. 3,357,485.
From a process point of view, not only the unfired residence time needs to be minimized, but also the pressure drop in the transfer line and TLE outside of the fire box must be reduced to improve the selectivity, because large pressure drops result in increased pressure and increased hydrocarbon partial pressure in the upstream pyrolysis tubes connected thereto, which adversely affects the pyrolysis reaction, as aforesaid. As discussed above, pressure drops are lower in the configuration of the subject invention than in a conventional apparatus.
Another problem associated with the use of TLE's concerns the temperature transition from the inlet which receives hot gas from the furnace, to the cooler exchange tubes, and the desirability of reducing the thermal stress on metal parts with such a steep thermal gradient. In U.S. Pat. No. 3,853,476 a steam purged jacket is employed in the inlet of the exchanger for this purpose. Applicants achieve this objective without the use of expensive steam by means of a novel structuring of the inlet of their heat exchanger unit.